Tunable coupler for mediating interactions between a two-level system and a waveguide from a decoupled state to the ultrastrong coupling regime
N. Janzen, Xi Dai, S. Ren, Jiahao Shi, Adrian Lupaşcu
Abstract
Two-level systems (TLSs) coupled to waveguides are a fundamental paradigm for light-matter interactions and quantum networks. We introduce and experimentally demonstrate a method to tune the interaction between a TLS, implemented as a flux qubit, and a transmission line waveguide from a decoupled state to a coupling strength that is a significant fraction of the TLS transition frequency, near the ultrastrong coupling regime. The coupling, controlled via magnetic flux, is described by a normalized coupling strength $\ensuremath{\alpha}$ that is measured to range between $6.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ and $2.19\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}$, with larger attainable maximum values predicted by a circuit model of the device. This system enables future investigations in the dynamics of the spin-boson model, microwave photonics, and relativistic quantum information.